This invention relates to catalytic cleaning systems for exhaust gases from internal combustion engines which have two exhaust lines and corresponding catalytic converters, each receiving exhaust from at least one combustion chamber.
The most effective device for cleaning the exhaust of internal combustion engines with spark-controlled ignition is a controlled three-way catalyst in which oxidizing and reducing reactions with various exhaust gas constituents occur simultaneously. The efficacy, or efficiency, of such catalysts is specified in terms of the conversion factor, which defines the extent to which those chemical reactions actually go forward in the catalyst as a percentage of the conversion of the specified gas constituents. This conversion factor varies with temperature and, in order for the catalyst to be fully effective, the catalyst temperature must exceed a minimum temperature, which is referred to as the start-up temperature. As the catalyst ages, the catalyst start-up temperature increases and, at the same time, its maximum attainable conversion factor decreases.
Thus, while the exhaust system must be arranged so that the catalyst start-up temperature is reached as soon as possible during operation of the engine, even after a cold start, on the other hand, it must also be arranged to limit the maximum temperature to which the catalyst is subjected, since excessive temperatures will more than proportionally accelerate aging of the catalyst. Accordingly, to ensure a high conversion factor of the catalyst over a long service life, the catalyst must be operated within a temperature range that becomes ever narrower with increasing service time, i.e., between a substantially constant upper temperature limit to minimize the effects of aging and a start-up temperature which increases with aging.
During normal operation of an internal combustion engine, especially one used in a motor vehicle, however, wide fluctuations of exhaust temperature occur. This variation in temperature prevents optimal catalyst performance, at least in certain modes of operation of the engine.
German Offenlegungsschrift No. 2 322 057 discloses an arrangement for catalytic afterburning of exhaust gases of a multicylinder internal combustion engine which allows for the low heat content of the exhaust immediately after a cold start by assigning an individual catalyst to each of the two banks of cylinders on the exhaust side and routing the exhaust from both banks of cylinders through one catalyst only, in response to signals from exhaust temperature sensors, until a certain exhaust temperature is achieved so that the catalyst will warm up quickly. Only after a preassigned minimum exhaust temperature has been reached will the exhaust be directed to the second catalyst as well. However, this arrangement has the disadvantage that the second catalyst, when cut in, has not yet been warmed up, and it must be heated by the hot exhaust gases to its start-up temperature so that, for a short period of time, the exhaust gases will pass through the second catalyst without having been cleaned. Furthermore, the first catalyst, which receives all of the exhaust gases immediately after a cold start, must be large enough to process the exhaust from all of the combustion chambers of the engine, since for a time that catalyst alone will be receiving the entire exhaust flow.
Nevertheless, this known arrangement with separate exhaust catalysts assigned to banks or groups of cylinders takes into account the temperature situation that arises in a cold start. Other arrangements having catalysts individual to banks of cylinders and having connections between the exhaust lines ahead of the catalyst are intended to create a pressure equalization ahead of the catalysts, as described in German Offenlegungsschrift No. 2 613 607, and/or to make the exhaust gas compositions in the lines leading to the two catalysts uniform, as described in German Offenlegungsschrift No. 3 740 238, without making allowance for special temperature conditions.
German Offenlegungsschrift No. 1 451 881 describes a device for catalytic conversion of exhaust gases from an internal combustion engine in which, after a cold start, two catalysts in series are charged with the exhaust gases from all combustion chambers. After heating of the exhaust gases, opening of a valve bypasses the smaller catalyst, which is closer to the combustion chambers and hence is heated more rapidly, so that only the larger catalyst, which is more distant from the combustion chambers, is employed for exhaust cleaning. This known device behaves favorably with respect to rapid exhaust cleaning after a cold start in two respects. First, the exhaust gases heat the smaller catalyst which is close to the combustion chambers comparatively rapidly. In addition, heating of the second, larger catalyst is simultaneously effected by the hot exhaust leaving the first catalyst and that heating may be further accelerated by means of heat-insulating jackets. Nevertheless, the lapse of time before adequate exhaust cleaning commences is comparatively long and, moreover, this arrangement does not meet the requirement for limiting the maximum temperature of the catalyst in order to prolong its life. On the contrary, it affords no possibility of limiting the temperature of the catalysts, especially since the catalysts and connections between them are heat-insulated for more rapid heating. This rapid heating is accomplished at the cost of a rapid aging of the catalysts.
A one-sided arrangement addressed exclusively to avoiding an overheating of the catalysts is disclosed in German Offenlegungsschrift No. 3 406 968. To ensure an optimum operating temperature for an exhaust catalyst, the exhaust in this arrangement is divided into two streams ahead of the catalyst and one of the streams is cooled while the other is not cooled. The two streams are reunited ahead of the catalyst in proportions selected in accordance with the temperatures of the streams. Thus, in the case of a cold start, the catalyst is charged exclusively with the uncooled partial exhaust stream but, because of the cold start, this stream is not yet hot.
Much the same assessment may be made of the arrangement for exhaust gas routing described in German Offenlegungsschrift No. 2 303 773. In this case, the supply of exhaust to a single catalyst takes place either by way of a thermally insulated exhaust line or by way of a cooling coil exposed to the airstream or placed in the flow of air from a fan. Any acceleration of the heating of the catalyst to its start-up temperature in a cold start is, at best, provided by the heat insulation of the exhaust line.
A more active solution than this is described in German Offenlegungsschrift No. 2 062 500. Here the exhaust gases, before they enter the catalyst, pass through an electric heater actuated before, during and/or after starting of the engine. This device requires a relatively large quantity of energy for the heating device, since that device must be designed to accelerate the heating of the single catalyst which receives all of the exhaust gases. Moreover, this arrangement does not satisfy the requirement for limiting the maximum catalyst temperature.
An entirely different principle is utilized in the arrangement described in German Offenlegungsschrift No. 2 210 031. In this arrangement, the ignition point is retarded and the throttle is opened wide during a cold start and during the usual idling operation of the engine following the cold start. Although increased retarding of ignition of the fuel-air mixture supplied to the combustion chambers of the engine correspondingly decreases the output of the engine, this is not important as long as the frictional resistance of the engine components is overcome sufficiently to maintain idling operation. The point is that with increasing retardation of ignition, the temperature of the engine exhaust rises, and the energy released by combustion is increasingly transformed into exhaust heat. A special advantage of this arrangement is that external energy, such as electrical energy from a battery in the case of a motor vehicle, is not required to accelerate the heating of the catalyst. Instead, the catalyst heating is effected only by varying operating parameters, i.e., the ignition point and the throttle setting, of the engine, utilizing the existing phases of engine operation. As soon as a load demand is placed upon the engine, the operating parameters are automatically restored to its value in normal operation, in particular, the travelling mode, i.e., the ignition point is advanced and the throttle opening is reduced according to the load demand. What this arrangement does not provide is a limitation of the maximum catalyst temperature. Moreover, the heating of the catalyst after a cold start is not optimized because of the use of a single and accordingly comparatively large catalyst.
As the foregoing analysis of the relevant prior art clearly shows, the art has been aware for decades of the twofold problem, i.e., the necessity for rapid heating of the catalyst and the requirement for limitation of the maximum catalyst temperature, and there have been many attempted solutions. Nevertheless, each of the prior art arrangements relates essentially to only one of the two problems.